52 Messrs, Hancock & Howse on Janassa Ijituminosa 



they are elongated, somewhat depressed, ovate, tapering a 

 little posteriorly, and have the surface divided into two well- 

 marked portions — an anterior scoop-like cutting-margin, and 

 a posterior ridged crushing-surface or disk, with a long de- 

 pressed root extending backwards (PI. II. figs. 2, 4, 5). 

 The scoop-like cutting-margin is considerably more than one- 

 fourth the entire length of the crown ; it projects upwards and 

 forAvards, and is smooth and concave, with the edge usually 

 obtuse and arched or a little sinuous from wear, but when 

 comparatively fresh is pretty regularly arched, and when 

 quite perfect is probably denticulated, if we may judge from 

 •the small lateral teeth. The crushing-surface or disk is elon- 

 gated, the sides being nearly parallel, though tapering to a 

 blunt point behind, the general form resembling that of a 

 lengthened shield. The surface is convex, and is covered 

 with about twenty close-set transverse ridges, imbricated for- 

 wards, and irregularly undulated, notched and tuberculated, 

 and arched forwards at the sides. 



The scoop-like cutting-margin and the crushing-disk we 

 shall call the upper surface, these being, in fact, the only 

 exposed portions, though in reality they represent the surface 

 that is usually considered the back of the tooth. The other or 

 opposed surface, which in ordinary cases would be called the 

 front, we shall name the under sui-fsice, because it is under- 

 most as the tooth rests on the jaw. The under surface, then, 

 presents a very peculiar appearance : it is divided into three 

 sharply defined, longitudinal, flattened areas or facets ; so that 

 in transverse section this side would show as half a hexagon. 

 The central area, which is divided from the two lateral areas 

 by a ridge or angle, is usually a little channelled. The back 

 of the scoop-like cutting-margin is also a little flattened at the 

 sides and centre. 



The root is a depressed process, longitudinally striated, 

 somewhat narrower than the crown, and about half its length ; 

 it originates in the under surface near to the posterior extre- 

 mity, and arches backwards and downwards. It is con- 

 sequently an extension, as it were, of the crown in a plane 

 below the crushing-disk. 



When seen in profile the primary teeth are observed to 

 assume a decided sigmoid curve, the anterior scoop-like cut- 

 ting-margin being turned rather abruptly in one direction, 

 and the posterior extremity of the crushing-disk and root in 

 the other or opposite direction (fig. 4). 



The large primary teeth, which hold a central position, are 

 symmetrical ; the smaller lateral ones, though they agree in 

 every other respect with the above, are more or less oblique, 



